1. Field of the Invention
The present invention relates to a driving apparatus of a piezoelectric vibrator used for a gyroscope, etc., more particularly, to a driving apparatus of a piezoelectric vibrator capable of being driven by a stable phase.
2. Description of the Related Art
FIG. 17 is a circuit constructional diagram showing driving means and detecting means of a piezoelectric vibrator for a conventional gyroscope; FIG. 18 is a perspective view showing a piezoelectric vibrator of a three-leg tuning fork type used for a gyroscope; FIG. 19 shows one constructional example of an end surface of a piezoelectric vibrator and is a front view thereof in the view of a direction of an arrow 19 in FIG. 18; FIG. 20 is a front view showing another constructional example of the end surface of the piezoelectric vibrator; and FIGS. 21(A) and 21(B) show diagrams using a conventional method of obtaining the median of a phase differential between two voltage outputs, FIG. 21(A) shows a case wherein a differential of amplitudes is equal to zero, and FIG. 21(B) shows a case wherein amplitudes have a differential.
As shown in FIG. 17, the conventional gyroscope comprises: a piezoelectric vibrator 1; driving means (AC drive signal source) 10 for supplying a drive signal to the piezoelectric vibrator 1; and detecting means 20 for detecting an output signal from the piezoelectric vibrator 1.
To start with, the piezoelectric vibrator 1 will be described. As shown in FIG. 18, the piezoelectric vibrator 1 is formed by adhering a piezoelectric material to both the front and back surfaces of a plane plate, which is made up of a constant-modulus material such as elinvar, alternatively by a plate material which is formed by a piezoelectric material such as PZT or crystal wholly. The piezoelectric vibrator 1 has three vibrating legs 1u, 1v and 1w which are formed in a forked manner at one end of the piezoelectric vibrator 1. As shown in FIG. 18, FIG. 19, and FIG. 20, pairs of drive electrodes 1a and 1b, a and b, and 2a and 2b are formed onto the front surfaces of the vibrating legs 1u, 1v, and 1w, so as to extend in parallel from one end portion to a base end portion. Pairs of output electrodes 1c and 1d, c and d, and 2c and 2d are also formed onto the back surfaces of the vibrating legs 1u, 1v, and 1w, respectively. An earth electrode G is formed in the middle of the output electrodes c and d on the back surface of the vibrating leg 1v as a center, so as to extend from one end portion to the base end portion.
Arrows in FIG. 19 indicate dielectric polarizing directions of the piezoelectric material at the three vibrating legs 1u, 1v, and 1w of the piezoelectric vibrator 1. The dielectric polarizing directions are the same at the vibrating legs 1u and 1w of the piezoelectric vibrator 1 on both right and left sides. The dielectric polarizing directions of the center vibrating leg 1v contrast with the vibrating legs 1u and 1w on the right and left sides horizontally and vertically, respectively (a differential polarizing type).
In the piezoelectric vibrator 1 of the differential polarizing type, if supplying the same drive signal S to the drive electrodes 1a and 1b, a and b, and 2a and 2b from the driving means (AC drive signal source) 10, a piezoelectric effect causes the vibrating legs 1u, 1v, and 1w to be vibrated to an X-direction serving as an array direction of the vibrating legs, as shown in FIG. 19.
A primary or multiple mode causes deformation vibration bending to the X-direction in the vibrating legs 1u, 1v, and 1w. The vibrating leg 1u and 1w on both sides are driven by the same phase. The vibrating leg 1v at the center is driven so that this phase is different from the vibrating legs 1u and 1w by .pi. (180.degree.), respectively. That is, when the vibrating legs 1u and 1w on both sides have an amplitude direction as a (+X)-direction at a certain point, the center vibrating leg 1v has an amplitude direction of a (-X)-direction.
As mentioned above, if setting the vibrating legs to a rotational system having a Z-direction with a vibrated state, Coriolis force works in a direction perpendicular to the vibrating direction (X-direction) to the vibrating legs, and the vibrating legs are vibrated to a Y-direction. With respect to a vibrating component due to the Coriolis force, the phases of the vibrating legs 1u and 1w on both sides are also opposite to the phase of the center vibrating leg 1v. When Coriolis force causes the vibrating legs 1u and 1w on both sides to have an amplitude component of a (+Y)direction at a certain point, the center vibrating leg 1v has an amplitude of a (-Y)-direction.
As shown in FIG. 20, in case of a piezoelectric vibrator (the same polarizing type) 1b such that all of the dielectric polarizing directions of the vibrating legs 1u, 1v, and 1w are formed to have the same direction, drive signal S1 and S2 having a differential phase of 180.degree. each other are supplied between adjacent drive electrodes on one vibrating leg, respectively. In other words, the drive signal S1 is supplied to the drive electrodes 1b and 2b in the (+X)-direction in the vibrating legs 1u and 1w on both sides, and the drive signal S2 is supplied to the drive electrodes 1a and 2a in the (-X)-direction therein. Contrarily, the d rive signal S2 is supplied to the drive electrode b in the (+X)-direction in the vibrating leg 1v, and the drive signal S1 is supplied to the drive electrode a in the (-X)-direction therein. As stated above, the drive signals S1 and S2 are supplied to the drive electrodes of the vibrating legs 1u, 1v, and 1w, respectively, thereby enabling the piezoelectric vibrator 1B to be vibrated similarly to the piezoelectric vibrator 1.
If setting the piezoelectric vibrator (the differential polarizing type) 1 or 1b (the same polarizing type) to any desired rotational system, current outputs I1 and I2 like sine waves with different phases are outputted between the earth electrode G and the output electrode c and between the earth electrode G and the output electrode d, respectively. A signal is outputted so that the median of the phase differential between the current outputs I1 and I2 is synchronized with a timing of a leading edge of the drive signal S. Properly speaking, the driving means 10 is feedback-controlled so that the drive signal S is synthesized with the median of the phase differential between the current outputs I1 and I2.
The next description turns to the operation of the driving means 10 and the detecting means 20. It is noted that it is assumed that when setting the phase differential between the current outputs I1 and I2 to .lambda., .lambda./2 as the median of the phase differential .lambda. is set to a reference point (0 deg) of the phase.
As shown in FIG. 17, the driving means 10 comprises: I/V (current/voltage) converting means 11; adding means 12; first phase shifting means 13; a coupling capacitor C1; binarizing means 14; second phase shifting means 15; gain varying means 16; and buffer means 17. The detecting means 20 comprises binarizing means 21 and phase differential detecting means 22.
In the piezoelectric vibrator 1, the output electrodes c and d of the center vibrating leg 1v are connected to the I/V (current/voltage) converting means 11 which is provided for the first stage of the driving means 10. The I/V (current/voltage) converting means 11 is constructed by an operational amplifier, etc. mainly, and comprises I/V converting circuits 11A and 11B, to which a resistor, a capacitor, and the like are attached externally around the operational amplifier, etc. The output electrode c of the piezoelectric vibrator 1 is connected to an input terminal 11a1 of the I/V converting circuit 11A, and the output electrode d is connected to an input terminal 11b1 of the I/V converting circuit 11B, respectively. The earth electrode G is connected to a reference potential (such as 0V).
The I/V converting circuits 11A and 11B convert into voltage outputs V1 and V2, the current outputs I1 and I2 like sine waves which are outputted from the output electrodes c and d. Note that this current/voltage conversion causes the voltage outputs V1 and V2 to be phase-delayed by -180 deg from the current outputs I1 and I2.
As shown in FIG. 17, output terminals 11a2 and 11b2 of the I/V converting circuits 11A and 11B are connected to the adding means 12. The adding means 12 comprises, for example, a resistor for dividing outputs of the I/V converting circuits 11A and 11B and a buffer circuit (not shown) having a high input impedance. The voltage outputs V1 and V2 are set to an additional voltage of V0 which is obtained by analog addition by the adding means 12. Incidentally, the phase is not shifted herein.
The first phase shifting means (analog phase shifter) 13 is provided for a post stage (third stage) of the adding means 12, and the additional voltage V0 is inputted to the first phase shifting means 13. The first phase shifting means 13 controls a peak value of the adding voltage V0 of the additional means 12 to simulate the median of the phase differential between the voltage outputs V1 and V2, and generates a reference signal (analog value) ref which is obtained by shifting the adding signal from the peak value by +90 deg. Therefore, the adding signal is shifted by [-180+(+90)=-90] deg from the reference point of the phase. It is to be noted that the reference signal ref is supplied to binarizing means 21c, which will be described hereinlater.
The coupling capacitor C1 is provided for a post stage (forth stage) of the first phase shifting means 13, and a DC component of the reference signal ref is cut. The first binarizing means 14 is provided for a post stage (fifth stage) of the coupling capacitor C1. The first binarizing means 14 converts the reference signal ref into a binarizing code (digital value) including signals of the "H" level and "L" level. Namely, an arbitrary threshold level (voltage) is set to a reference, and a reference signal V0' is converted into the "L" level signal if the reference signal V0' is equal to or more than the threshold level, and the reference signal 0V' is converted into the "H" level signal if it is equal to or less than the threshold level. Consequently, the phase is delayed by -180 deg in the first binarizing means 14. This results in delay by [-90+(-180)=-270] deg, in view of the reference point of the phase.
The second phase shifting means (digital phase shifter) 15 is provided for a sixth stage. The second phase shifting means 15 functions to shift the phase of the first binarizing means 14 by -90 deg. Therefore, the output of the second phase shifting means 15 is shifted by [-270+(-90)=-360=0] deg, namely, to the reference point of the phase. The output of the second phase shifting means 15 is amplified to a predetermined amplitude by the gain varying means (driving circuit) 16 provided for a post stage (seventh stage), and converted into the drive signal S (drive power) like a sine wave again. The drive signal S is a signal having a phase differential of 0 from the reference point of the phase, and supplied to the drive electrodes 1a, 1b, a, b, 2a, and 2b of the piezoelectric vibrator 1, by way of the buffer means 17.
The detecting means 20 is provided with the second binarizing means 21 for binarizing the voltage outputs V1 and V2 of the I/V converting means 11. The second binarizing means 21 comprises three binarizing circuits 21a, 21b, and 21c. The voltage output V1 of the I/V converting circuit 11A is inputted to the binarizing circuit 21a among the binarizing circuits 21a and 21c, and the voltage output V2 of the I/V converting circuit 11B is inputted to the binarizing circuit 21b. The binarizing circuits 21a and 21b convert the voltage outputs V1 and V2 of the I/V converting means 11 into digital outputs D1 and D2 having the "H" signal and "L" signal at a predetermined threshold level, respectively. Incidentally, the voltage outputs V1 and V2 are shifted by -180 deg in the binarizing circuits 21a and 21b. Accordingly, in view of the reference point of the phase, the phase differential is equal to [-180+(180)=-360=0] deg.
The reference signal ref, which is phase-shifted by the first phase shifting means 13 at the third stage of the driving means 10, is inputted to the remaining binarizing circuit 21c among the second binarizing means 21, and converted into a digital reference signal D.sub.ref having the "H" level signal and "L" level signal at a predetermined threshold level (voltage). In this case, a phase of the digital reference signal D.sub.ref is equal to [-90-(180)=-270] deg (=+90 deg) for the reference point of the phase. Namely, this case is set to generate a phase differential of 90 deg between the digital outputs D1 and D2 and the digital reference signal D.sub.ref.
The phase differential detecting means 22 comprises: a set of Exor gates 23 and 24; low-pass filters 25 and 26; and differential amplifying means 27. An exclusive OR between the digital output D1 and the digital reference signal D.sub.ref is obtained in the first Exor gate 23. An exclusive OR between the digital output D2 and the digital reference signal D.sub.ref is obtained in the second Exor gate 24. The outputs are integrated and smoothed by the low-pass filters 25 and 26, converted into DC voltage values, and the differential amplifying means 27 further detects an angular velocity output Vout proportional to Coriolis force.
The thus-detected angular velocity output Vout is further value-integrated by integrating means (not shown) and an angle is obtained, thereby using the angle as an internal signal of the gyroscope.